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The ArcelorMittal Orbit: Twist and shout

The ArcelorMittal Orbit in the Olympic park is being built to ‘arouse the curiosity and wonder of Londoners’. And the most curious thing of all is how this spiralling confusion of red steel actually stands up

orbit

When the ArcelorMittal Orbit was unveiled last March, it was variously described as looking like “the Eiffel Tower after a nuclear attack”, “the Wembley arch after a design review with King Kong” and “a cluster of mobile phone masts erected by builders on crack”. The reason for this controversy was down to the then culture secretary Tessa Jowell and London mayor Boris Johnson deciding that something was needed to give the Olympic park its missing wow factor - to “arouse the curiosity and wonder of Londoners and visitors”, in Johnson’s words. They held a design competition which was won by sculptor Anish Kapoor and engineer Cecil Balmond. And so was born the Orbit: a structure that looks like a radio mast that has melted and slumped into a truncated, convoluted form.

Love it or loathe it, the Orbit is undoubtedly an intriguing structure. According to its creator Cecil Balmond, the about-to-fall-down look is intentional. “Both Anish and I have this agenda to find forms that are intriguing and ambiguous; where the structure is not obvious,” Balmond explains. “This has ambiguity and a structural form on the edge of instability. It isn’t unstable but appears vulnerable.”

The Orbit is rapidly taking shape in east London, so how has the design team turned this bizarre concept into something that stands up and can be realised on a building site?

Swinging up and down

Believe it or not, the Orbit is a single tube that wraps around itself to create this complex-looking form (see box, right). The tube starts as a bell-shaped form 38m in diameter, which helps to give the structure stability and provides space at its base for visitors, as well as the lift shaft used to get them to a two-storey viewing platform, 80m up. Above the lift shaft, this structure narrows to a 4m-wide section and loops over and around itself, before flaring out again at the end to support the drum-shaped viewing platform. The tube twists like a rope which gives it a jerky outline, adding to the general visual confusion.

Despite its convoluted appearance, the structure isn’t particularly complicated. Balmond has been responsible for the structural design of two Serpentine pavilions and says these gave him more sleepless nights than the Orbit. The line of the Orbit touches the ground three times, forming a stable, tripod-like structure. The team has nicknamed the initial section with the lift shaft “the octagon” and the narrow segments “the intestines”. These elements are connected as they pass each other which further enhances stability.

Like any tall structure, the Orbit will move around in the wind. As people will be able to ascend the structure, Arup had to ensure they wouldn’t get sick from the swaying. So a tuned mass damper will be placed on top of the lift shaft to help dampen vibration and enable fine tuning of the structure’s movement patterns. “Because tuning buildings is a bit of a black art, if there are any discrepancies between the calculated and actual frequencies, we can fix it,” explains Richard Henley, the project director for principal designer Arup.

The structure is made from steel tube and consists of 4m-high sections joined together as a series of rings. Each ring is twisted relative to its neighbour so the interconnecting struts form a triangulated structure. The first section consists of eight-sided rings to create enough space for accommodation at the base and the lift shaft. One seven-sided section is needed, as this is the right width for the lift doors, while the intestines are made up from square sections. These are rotated at 45º relative to their neighbour to create the triangular structure and the twisting, rope-like appearance.

Constructing the Orbit

Unsurprisingly, all the steel tube needed for the project is being supplied by project sponsor and steel giant ArcelorMittal. However, the six tubes that form each star-shaped node (see image, right) - which, in turn, form the rings - are fabricated in steel specialist Watson Steel’s factory in Bolton, before being joined together onsite. The nodes were complex to fabricate because the six tubes don’t butt up neatly against each other but are offset to create the curving nature of the structure. The answer was to use flat steel plates at the nodes. “To simplify it, we decided to go for this separating plate, which allows the tubes to come together without butting up, and hides the overlap,” says Holger Falter, the Arup associate who has done much of the detailed structural design.

However, a way still had to be found to connect the stars to each other. Initially a joint hidden by a cover plate was considered but was deemed too expensive. A simple flange connection was much cheaper but would be highly visible. “We thought very hard about this,” says Balmond. “We concluded that if the form is very strong then the practical details that make the form work don’t matter.”

The flange connections had another advantage. The structure may be relatively simple but it has to be built with extreme precision to ensure all the elements fit together onsite. The node rings are fabricated to a tolerance of +/- 1mm, and the flanges could also be machined in the factory to very fine tolerances to ensure everything fitted.

The construction team only had a few yards to travel as most of them were working on the Olympic stadium next door. Chris Keenan was Sir Robert McAlpine’s construction manager on the stadium and is now project manager for the Orbit. He says the timescale for constructing the Orbit was incredibly tight - planning permission was granted in August 2010 and the team were onsite by the end of September.

It was crucial to ensure the abutments for the initial ring sections were in precisely the right place. Steelwork can be made to very precise tolerances but it is much harder to get the foundations right, as concrete is prone to shrinkage. This could potentially have thrown the whole structure out, but an unusual solution was adopted: rather than casting bolt fixings into the foundations and bolting the first superstructure elements to these, empty pockets were left in the slab ready to receive specially made steel-to-concrete connections. These were fixed to the first steel sections of the superstructure, with the idea they would be precisely positioned before concrete was poured around the connections to attach them to the foundation.Three fixing points were needed for each leg of the main structure. If conventional bolted fixings had been used, the superstructure could have been bolted to each fixing point individually, but here all three superstructure elements had to be positioned together. “We had to hold each piece in position with a crane, which meant we had to use three cranes together,” says Keenan.

Most of the job consists of the steel structure. It makes up 60% of the project value compared to 15% for a typical building. Watson Steel is responsible for fabricating the steel elements, erecting them on site and painting the finished structure. At the moment there are six steel erectors onsite but, according to Watson Steel’s construction manager John Calland, most of the structure has been erected by three steel workers and three crane drivers.

The lower sections came to site as large star-shaped elements that were simply lifted into position by crane and bolted together by steel workers on cherry pickers. The plan was to erect the main central section together with the spiral stairs and put up the intestine loops afterwards, but as the stair was delayed the team changed tack and have erected the main section and the intestines at the same time.

Problem solving

The cherry pickers’ limit is 40m, so once the structure was up to this height a different approach was called for. The team decided to pre-assemble the elements on the ground. “The idea was to build as big as possible on the ground to reduce the risks of working at height,” explains Calland. The elements making up each ring of the central section are assembled and painted on the ground with only the connections painted at height. The rings are craned up and bolted together at height, with the cherry picker sitting on a platform on top of the lift shaft. Three 4m-high rings are added to the tower, then another section is added to the lift shaft to raise it up by 12m, ready for the next three rings.

This might sound simple but the team has to contend with three different challenges. First, the dead weight of the structure means that it leans over by 170mm more than if there was no gravitational pull. This is due to more weight being concentrated on the south side. In practical terms, this means the structure starts off being built in the wrong position. As more elements are added to the structure, it progressively leans over toward its final position.

Watson is working with two computer models, one showing where the structure would be with gravity and the other without. The challenge is determining where the structure should be at each stage of construction. “When you are building it you are between two models,” says Calland. “We had to analyse all the different stages to see where it should be at that point in time.”

As each element is fabricated for a fully assembled scenario, but the structure isn’t in its final position, they don’t always match up during construction. “There is quite a lot of pulling and pushing to get the elements in position, which can be quite difficult,” says Keenan. “Because we are working to a global geometry we never change the elements to fit. If we changed anything it would cause problems further on in the build.”

The second challenge is the weather. Keenan says June, July and August have been particularly windy. “Right now the weather is our biggest challenge as the structure is so high and exposed,” he says. “A traditional tower would have lots of floors that you could work on but here there is nothing between the ground and the top.” As the tower progresses upwards the conditions become even more important. Each of the intestines forms a big loop that swoops up, then down towards the ground. These will be craned up in complete sections and bolted onto the structure by workers in man cages suspended by cranes. This can only be done in comparatively still conditions.

The third challenge is the very restricted site and linear nature of this job. “One of the main challenges is co-ordinating the different activities as these are mutually exclusive,” says Calland. For example, the painters can’t paint the steel connections on the tower while the steel erectors are working above. When wind stops work at height, attention moves to assembling and painting the sections on the ground.

The plan is to get the two big intestine loops in place and the structure finished in October so work can start in earnest on the observation deck. The project is scheduled to be completed by March 2012, and Londoners will then be able to decide if the ArcelorMittal Orbit is indeed a fitting emblem for the Olympics and their city.

What is the Orbit all about?

When Anish Kapoor and Cecil Balmond answered Boris Johnson’s call to design an icon for the Olympic park, they didn’t want to repeat the past, but they looked at earlier structures such as the Eiffel Tower, the never-realised Tatlin’s Tower, designed for Petrograd, and depictions of the Tower of Babel.

“All tall towers have the same language in that they are continuously connected from the ground up and have these triangular forms,” says Balmond.

The inspiration was the idea of a line orbiting around a single point in space rather than a conventional, linear form. This evolved into a line orbiting around two fixed points to form a figure of eight. If the orbiting lines were attached where they passed each other this would give the structure rigidity. Because the orbiting lines touch the ground a stable tripod structure could be created. This gave the pair “an ambiguous structural form on the edge of instability,” says Balmond. He describes this as a “paradigm for London”: a non-linear flux due to its mix of cultures and constantly changing landscape.

Whether the public will get this concept when they see the Orbit is a moot point. Originally the Orbit was to be 180m tall and support three viewing platforms, with secondary, braid-like lines circling the primary structure. Budgetary constraints saw off the braids and multiple viewing platforms, while the height of the structure was reduced to 115m. As it is, the sculpture consists of 1,250 tonnes of steel and will cost £23m.

Taking a view

Enjoying the view from a tall, steel structures is hardly a new idea - the Eiffel Tower was built for this purpose and predates the Orbit by 112 years. Up to 300 people can be accommodated on the Orbit’s two-storey drum-shaped viewing gallery 80m up in the air. Balmond says he wanted visitors to enjoy the Orbit as a “narrative experience”.

Visitors enter the trumpet-shaped base where there is a cone above their heads, meaning they can’t see the top of the structure. They step into a lift which takes them up to the viewing platform. In addition to the views over the Olympic park and London beyond, they can look through an opening in the centre of the viewing platform to get a bird’s-eye view of the structure below. The second storey includes strategically placed mirrors which, according to Balmond, “bring the sky to your feet”, meaning visitors get a full 360º experience from the Orbit.

Once they tire of the view, they can either descend via the lift or take a staircase which spirals around the structure. “This was the hardest element to integrate into the structure as you couldn’t have people throwing things down,” says Balmond.

Architect Ushida Finlay was engaged to ensure that the structure, services and landscaping worked as an integrated whole and that the Orbit conformed with Building Regulations and planning requirements. The firm’s main visual contribution is a screen wrapped around the stairs which stops anything - or anyone - falling off the structure. Original visualisations show a simple, red-painted escape stair but later images show that the spiral stair is a major visual element. The screen is made from stainless steel sheeting which has apertures cut into it. The sheet is more solid at its base to make people feel more secure and to catch dropped items. The mesh is more open above hand-rail level so people can still enjoy the view.